1. Field of the Invention
The present invention relates to a printed circuit board with embedded capacitors therein, and a process for manufacturing the printed circuit board. More particularly, the present invention relates to a printed circuit board with embedded capacitors therein in which the embedded capacitors are formed by applying a photosensitive insulating resin to a printed circuit board (PCB) inner layer and applying a high dielectric polymer capacitor paste thereto, and a process for manufacturing the printed circuit board.
2. Description of the Related Art
Common discrete chip resistors or discrete chip capacitors have been mounted on the surfaces of conventional printed circuit boards (PCBs). Recently, printed circuit boards with embedded passive devices such as resistors, capacitors, etc. therein are developed.
Such printed circuit boards with embedded passive devices therein are manufactured by a process comprising inserting passive devices such as resistors, capacitors, etc., into an inner or outer layer of a substrate using novel materials (substances) and processes, thereby replacing the functions of conventional chip resistors and chip capacitors. That is, the printed circuit boards with embedded passive devices therein include passive devices, e.g., capacitors, buried in the inner or outer layer of the substrate. The term “embedded capacitors” used herein refers to capacitors as passive devices which are mounted as parts of a printed circuit board, irregardless of the size of a substrate. The substrate is referred to as “embedded capacitor PCB”. The most important advantage of the embedded capacitor PCB is that since the capacitors are mounted as parts of the printed circuit board, there is no need to mount the capacitors on the surfaces of the printed circuit board.
On the other hand, recent techniques for manufacturing the embedded capacitor printed circuit board are largely classified into the following three techniques:
The first technique is a method for manufacturing polymer thick film type capacitors comprising application of a polymer capacitor paste and thermal curing (that is, drying) of the paste. Specifically, according to this method, the embedded capacitors are formed by applying the polymer capacitor paste to the printed circuit board inner layer, drying the polymer capacitor paste, printing a copper paste on the resulting printed circuit board to form electrodes and drying the copper paste.
The second technique is a method for manufacturing embedded discrete type capacitors comprising coating a ceramic filled photo-dielectric resin onto a printed circuit board. The related patent is owned by Motorola Inc. Specifically, according to this method, the discrete capacitors are formed by coating the photo-dielectric resin containing ceramic powders onto both surfaces of a substrate, laminating copper foils thereto to form respective top electrodes and bottom electrodes, forming circuit patterns thereon, and etching the photo-dielectric resin.
The third technique is a method for manufacturing power distributed decoupling capacitors comprising inserting dielectric layers having a capacitance characteristic into the printed circuit board inner layer, thereby replacing decoupling capacitors mounted on the surfaces of the conventional printed circuit boards. The related patent is owned by Sanmina Corp. Specifically, according to this method, power distributed decoupling capacitors are formed by a process comprising inserting dielectric layers composed of power electrodes and ground electrodes into the inner layers of the printed circuit board.
A variety of processes are developed based on the three techniques discussed above. Procedures for carrying out the processes are different from each other. Since a market for the printed circuit boards with embedded capacitors therein is small, the standardization of these techniques is not yet achieved and trials to develop commercially available processes are still ongoing.
Hereinafter, printed circuit boards with embedded capacitors therein and the processes for manufacturing the printed circuit boards according to the prior arts will be explained in more detail with reference to the accompanying drawings.
The first prior art is explained with reference to FIGS. 1a to 1e. 
FIGS. 1a to 1e are sectional views showing a process for manufacturing a printed circuit board with embedded polymer thick film type capacitors therein according to the first prior art. Referring to FIGS. 1a to 1e, the printed circuit board with embedded polymer thick film type capacitors is manufactured by a process comprising application of a polymer capacitor paste and thermal drying (that is, curing) of the paste.
In step 1, a dry film is coated onto a copper foil of a PCB inner layer 42 composed of FR-4, and then subject to light exposure and development. The copper foil is etched to form copper foils 44a and 44b for positive (+) electrodes, copper foils 43a and 43b for negative (−) electrodes and clearances formed therebetween (FIG. 1a).
In step 2, a capacitor paste, a polymer containing ceramic powders of high dielectric constant, is applied to the copper foils 43a and 43b for negative (−) electrodes using a screen-printing manner, and is then dried or cured (FIG. 1b). The screen-printing manner refers to a printing method in which a medium, e.g., ink, is passed through a stencil screen using a squeezer to form a pattern on a substrate surface.
At this step, screened capacitor pastes 45a and 45b cover the copper foils 44a and 44b for positive (+) electrodes, the copper foils 43a and 43b for negative (−) electrodes and the clearances formed therebetween.
In step 3, a conductive paste such as silver and copper paste is applied to the resulting structure using a screen-printing manner to form positive (+) electrodes 46a and 46b, and then dried or cured (FIG. 1c).
In step 4, the capacitor layer formed on a copper foil 41 of the PCB inner layer 42 formed through the previous steps 1 to 3 is inserted between insulators 47a and 47b, and then the three layers are laminated to each other (FIG. 1d), wherein reference numerals 48a and 48b designate copper foil layers attached onto insulators 47a and 47b, respectively.
In step 5, through-holes (TH) and laser blinded via holes (LBVH) 49a and 49b are formed on the laminated product. The capacitors present in the interior of the laminated product are connected to corresponding positive (+) terminals 51a and 51b and negative terminals 50a and 50b of IC chips 52a and 52b through via the through-holes (TH) and the laser blinded via holes (LBVH) 49a and 49b, respectively. Accordingly, the capacitors function as embedded capacitors (FIG. 1e).
One of the problems concerning the first technique of the conventional techniques discussed above is the occurrence of cracks of the capacitor pastes 45a and 45b at ends of the positive (+) electrodes 46a and 46b. 
FIGS. 2a and 2b are sectional views illustrating problems of the printed circuit board manufactured by the process shown in FIGS. 1a to 1e, respectively.
Referring to FIG. 1b, when the capacitor pastes 45a and 45b are printed on the copper foils 43a and 43b for negative (−) electrodes and dried in step 2 above, cracks (C) occur as shown in FIG. 2a. The occurrence of cracks (C) results from the thickness of the copper foils for negative (−) electrodes. The copper foils of the PCB inner layer commonly have a thickness of ½ oz (18 μm) or 1 oz (36 μm). Since the thickness of the printed capacitors is about 10 μm, cracks occur at ends of the copper foils for negative (−) electrodes. The cracks also cause problems that when the copper pastes 45a and 45b connected to the copper foils 44a and 44b for positive (+) electrodes are printed, an interlayer short between the negative (−) electrodes and the positive (+) electrodes is poor.
In addition, the second problem concerning the first technique of the conventional techniques discussed above is non-uniform insulation distance between a first layer and a second layer shown in FIG. 1e. 
When the embedded capacitors 45a and 45b are formed in accordance with the steps of FIG. 1a to 1e and the insulators 47a and 47b are laminated as shown in step 4, the difference between the insulation distance from the capacitors to a first layer (designated by A) and the insulation distance from a second layer (designated by B) or a third layer in the inner layer core to the first layer, becomes large (FIG. 2b). For example, when the insulators have a thickness of 80 μm, the insulation distance (A) from the copper power electrodes 46a and 46b of the inner layer capacitors to the first layer is 20˜30 μm, while the insulation distance (B) from the FR-4 core 42 or the copper foils of the second layer to the first layer is 60˜70 μm. The difference between the insulation distances is because both the capacitor pastes 45a and 45b and the copper power electrodes 46a and 46b have a thickness of 10˜15 μm. The difference in the insulation distances leads to poor impedance of signal circuits in the first layer and the second layer.
Furthermore, the third problem concerning the first technique of the conventional techniques discussed above is the presence of impurities occurring during printing and drying of the capacitor pastes 45a and 45b and copper pastes 46a and 46b. 
As shown in FIG. 1b, after the capacitor pastes 45a and 45b are printed in a thickness of 10˜15 μm, dried at 150° C. or higher for 30˜90 minutes, the copper pastes 46a and 46b are printed thereon and dried. At this time, impurities occur during the printing. When the printed copper pastes 46a and 46b are dried, a number of voids are formed due to the impurities. The voids lead to a poor interlayer short between the positive (+) electrodes and the negative (−) electrodes when the copper pastes 46a and 46b connected to the copper foils 44a and 44b for positive (+) electrodes are printed on the capacitor pastes 45a and 45b and then dried.
Next, the second technique will be explained with reference to FIGS. 3a to 3c. 
FIGS. 3a to 3f are sectional views showing a process for manufacturing a printed circuit board with embedded discrete capacitors formed by coating a photo-dielectric resin in accordance with a prior art. The embedded discrete type capacitors are formed by a process comprising coating the ceramic filled photo-dielectric resin onto a printed circuit board. The process will be explained with reference with U.S. Pat. No. 6,349,456 issued to Motorola Inc.
In step 1, a dielectric layer 14 containing ceramic powders is coated onto a substrate 10 on which a conductor layer 12 is formed, and then subjected to light exposure and thermal drying (FIG. 3a).
In step 2, a laminate copper foil 16 is applied to the surface of the dried dielectric layer 14 (FIG. 3b). The reference numeral 18 designates a sacrificial layer tin-plated onto the copper foil 16 to use as a copper etching resist.
In step 3, a dry film is laminated to the sacrificial layer 18, and then exposed to light and developed. The sacrificial layer 18 and the copper foil 16 are etched to form top electrodes 20 (FIG. 3c).
In step 4, the dielectric layer 14 underlying the top electrode 20 is exposed to light, and then the resulting dielectric layers 22 are etched. The top copper electrodes 20 serve as photomasks for the dielectric layer 14 (FIG. 3d).
In step 5, the copper foil 12 underlying the etched dielectric layers 22 is etched to form bottom electrodes 24 (FIG. 3e).
In final step 6, the capacitor layer 32 formed through the previous steps 1 to 5 is inserted between insulators 26, and then metal layers are laminated thereon (FIG. 3f).
Thereafter, the capacitors 32 present in the interior of the laminated product are connected to corresponding power terminals and ground terminals of IC chips mounted on the exterior of the laminated product through via the through-holes (TH) and the laser blinded via holes (LBVH), respectively.
The first problem concerning the second technique of the conventional techniques discussed above is high manufacturing cost.
That is, in order to manufacture the embedded discrete type capacitors by coating the ceramic filled photo-dielectric resin 14 onto the substrate, both the top electrode 20 and the bottom electrode 24 must be formed. In this case, many steps, which greatly increase manufacturing cost, are required. In addition, since the technique comprises printing the photo-dielectric resin 14 on the entire surface of the bottom electrodes 12 (a copper foil layer), exposing to light, and etching to remove regions responded to light, more of the photo-dielectric resin than expected is required even to manufacture a small number of the embedded capacitors. Accordingly, the technique is economically disadvantageous in terms of considerable loss of raw materials.
The second problem concerning the second technique of the conventional techniques discussed above is interlayer shorts between the bottom electrodes 24.
Specifically, the laminate copper foil 16 is circuit-formed to form the top electrodes 20. When the photo-dielectric resin 14 underlying the copper foil 16 is exposed to light, followed by etching using an etchant to remove regions responded to light, the width of the removed regions may be small. The small width carries a risk that the unetched dielectric resin 14 may remain at the surface of the copper foil 12. This is because when the photo-dielectric resin 14 is thermally cured (110° C., 60 min.), a photosensitive agent present in the photo-dielectric resin 14 is not completely reacted. In particular, the photo-dielectric resin 14 located in the lower copper foil 12 may be unetched, thus resulting in unetched lower copper foil 12. Finally, interlayer shorts between the bottom electrodes 24 may occur.
Next, the third conventional technique will be explained in more detail with reference to FIGS. 4a to 4c. 
FIGS. 4a to 4c are sectional views showing a process for manufacturing a printed circuit board with embedded capacitors by inserting a dielectric layer having a capacitance characteristic, in accordance with a prior art. The embedded capacitors are formed by a process comprising inserting a separate dielectric layer having a capacitance characteristic into the printed circuit board inner layer. The embedded capacitors thus manufactured can replace decoupling capacitors mounted to the surface of the substrate. The process will be explained with reference with U.S. Pat. Nos. 5,079,069, 5,261,153 and 5,800,575 issued to Sanmina Corp.
In step 1, a dry film is deposited on a high electric copper coated laminate 61 between copper foils 63a and 63b, and subjected to light exposure and development. Then, the copper foils 63a and 63b are etched to form power electrodes and clearances (FIG. 4a).
In step 2, the resulting structure is inserted between insulators 64a and 64b, and then outer-layer copper foils 65a and 65b are laminated thereto (FIG. 4b).
In step 3, the capacitors present in the interior of the laminated product are connected to corresponding power terminals and ground terminals of IC chips 68a and 68b mounted on the exterior of the laminated product through via through-holes (TH) and laser blinded via holes (LBVH), respectively. The capacitors serve as power distributed decoupling capacitors (FIG. 4c). Reference numerals 67a and 67b designate clearance between a ground electrode and a power electrode, respectively. When the through-holes and laser blinded via holes are formed between the ground electrode and the power electrode, they are spaced sufficiently apart from the ground electrode and the power electrode not to contact one another.
The first problem concerning the third technique of the conventional techniques discussed above is low capacitance due to low dielectric constant of the embedded capacitor layer.
Specifically, in the case of a thin film type having a thickness of 10˜50 μm as shown in FIG. 4a, FR-4 (manufactured by Sanmina Corp.) having a thickness of 25 μm or 50 μm as a dielectric material is disposed between the copper foils used as power electrodes and ground electrodes. The FR-4 has a dielectric constant of 4˜5 and a capacitance value per unit area of 0.5˜1 nF/in2. Since the capacitance value of the FR-4 is considerably low, compared to that of currently used decoupling discrete chip capacitors (100 nf/in2), there are many a limitation in manufacturing embedded capacitors.
The second problem concerning the third technique of the conventional techniques discussed above is an increase in the thickness of a printed circuit board due to insertion of embedded capacitor layers.
Specifically, in order to manufacture a printed circuit board with embedded capacitors therein having a high capacitance using the FR-4 dielectric material, the insertion of a large number of embedded capacitor layers, which increases in thickness of the printed circuit board to be manufactured, is required. Accordingly, the technique is economically disadvantageous in terms of increased manufacturing cost.
The third problem concerning the third technique of the conventional techniques discussed above is the occurrence of interlayer shorts and cracks between power electrodes and ground electrodes. Specifically, in the case of using a thin dielectric material to obtain a high capacitance value, during dry film lamination and circuit formation of an upper copper foil for power electrodes and a lower copper foil for ground electrodes, as shown in FIG. 5, interlayer shorts and cracks between the power electrodes and the ground electrodes occur.
FIG. 5 is a sectional view illustrating a problem of a printed circuit board with embedded capacitors therein manufactured by a process comprising inserting a dielectric layer having a capacitance characteristic, in accordance with a prior art. In a printed circuit board with embedded capacitors therein, the capacitors being formed by inserting a dielectric layer having a capacitance characteristic in accordance with a prior art, interlayer shorts (designated by G) and cracks (designated by F) between power electrodes 92 having a thickness of 18˜35 μm and ground electrodes 93 on a high dielectric layer 91 having a thickness of 8˜10 μm occur, as shown in FIG. 5.
On the other hand, capacitance is dependent on the area and thickness of a capacitor, and is given by the following equation 1:                     C        =                              ɛ            r                    ·                                    ɛ              0                        ⁡                          (                              A                D                            )                                                          Equation        ⁢                                   ⁢        1            wherein ∈r is the dielectric constant of a dielectric material, ∈o is a constant of 8.885×10−8, A is the surface area of the dielectric material, and D is the thickness of the dielectric material. That is, in order to obtain high capacity capacitors, the dielectric constant of a dielectric material must be high, the thickness of the dielectric material must be small, and the surface area of the dielectric material must be large. In addition, when the thickness is 10 μm, the capacitance of the bimodal polymer ceramic composite is 5˜7 nF/cm2.
For example, according to U.S. Pat. No. 6,274,224 issued to 3M Innovative Properties Co., a thin film type dielectric material is disposed between the copper foils used as power electrodes and ground electrodes. The dielectric material is a composite of high dielectric BaTiO3 ceramic powders and a thermosetting epoxy resin or polyimide and has a thickness of 8˜10 μm. Although the dielectric material has a relatively high capacitance (10 nF/in2) per unit area, there are problems of interlayer shorts and cracks between power electrodes and ground electrodes due to low thickness of the dielectric material.